pgsql: Cascading replication feature for streaming log-based replicatio

Cascading replication feature for streaming log-based replication.
Standby servers can now have WALSender processes, which can work with
either WALReceiver or archive_commands to pass data. Fully updated
docs, including new conceptual terms of sending server, upstream and
downstream servers. WALSenders terminated when promote to master.

Fujii Masao, review, rework and doc rewrite by Simon Riggs

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5286105800c7d5902f98f32e11b209c471c0c69c

Modified Files
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doc/src/sgml/config.sgml | 127 ++++++++++------
doc/src/sgml/high-availability.sgml | 62 ++++++++-
src/backend/access/transam/xlog.c | 265 +++++++++++++++++++--------------
src/backend/postmaster/postmaster.c | 20 +++
src/backend/replication/basebackup.c | 5 +
src/backend/replication/syncrep.c | 7 +
src/backend/replication/walreceiver.c | 7 +-
src/backend/replication/walsender.c | 108 ++++++++++---
src/include/access/xlog.h | 6 +-
src/include/replication/walsender.h | 5 +-
10 files changed, 423 insertions(+), 189 deletions(-)